Human respiratory syncytial virus (HRSV) is the most important viral cause of respiratory disease in very young children. Currently there are no vaccines available. Previous studies have demonstrated the ability of subunit vaccines based on the HRSV fusion protein, F, to prevent virus replication in the lung. However, parenteral administration of these vaccines does not prevent virus replication in the upper respiratory tract, which is critical to reducing transmission of the virus. We propose to develop a recombinant mucosal immunogen containing the immunogenic and protective properties of the viral F protein and the adjuvant properties of the cholera toxin B subunit to induce a local immune response in the respiratory tract and to protect both the upper and lower respiratory tracts against virus infection. An expression vector in which the toxic A1 portion of the cholera toxin is replaced by the sequence encoding the ectodomain of the F1 subunit will be constructed. The chimeric protein will be expressed in E. coli, purified and used as a mucosal immunogen. Following intranasal immunization of mice, both the local and systemic humoral and cell-mediated immune responses will be characterized. The immunogen will be tested for its ability to protect both the upper and lower respiratory tract from viral challenge. Since earlier studies have implicated parenteral immunization with purified F protein in the development of pulmonary disease upon subsequent virus infection, we will also assess immunized animals for lung disease following viral challenge and characterize the pulmonary T lymphocyte population in these animals. These studies will contribute to our understanding of the mucosal immune response to HRSV and the efficacy of mucosal immunization with recombinant immunogens containing HRSV proteins. The long-term goal of this work is the development of innovative vaccines designs, including improved mucosal adjuvants and delivery systems and the development of effective vaccines for HRSV and other respiratory pathogens. Specific Aim 1: The development of a recombinant immunogen possessing the adjuvant properties of the cholera toxin B subunit and the immunogenic and protective properties of the HRSV F protein. Specific Aim 2: Characterization of the humoral and cell-mediated immune response to the recombinant immunogen in the respiratory tract of mice. Specific Aim 3: Analysis of the capacity of the recombinant immunogen to induce a protective response against challenge with live virus without the development of immunopathology in the mouse lung.